The present invention relates to a process for preparing 2-methyl-3-(4-tert-butyl-phenyl)propanal with high para-isomeric purity, and also to a process for preparing 4-tert-butylbenzaldehyde with high para-isomeric purity.
2-Methyl-3-(4-tert-butylphenyl)propanal is in particular used as lily-of-the-valley fragrance, and the protected trademarks Lysmeral®, Lilial®, or Lilestralis® are often used to refer to the material in this connection. Because of the production process used, marketed 2-methyl-3-(4-tert-butylphenyl)propanal of the formula (I) always also comprises 2-methyl-3-(3-tert-butylphenyl)propanal of the formula (II).

There are a number of known processes that can give compound (I) or a mixture of compounds (I) and (II). By way of example, the specification EP 0 045 571 teaches that the compounds (I) and (II) can be accessed starting from benzaldehyde via condensation with propanal and subsequent hydrogenation of the α,β-unsaturated aldehyde to give 2-methyl-3-phenyl-1-propanol.

The ratio between the corresponding para-isomeric and the meta-isomer can be adjusted to a certain extent by controlling the reaction conditions during introduction of the tert-butyl moiety into the 2-methyl-3-phenyl-1-propanol. The two isomeric alcohols are then dehydrogenated to give the target compounds (I) and (II).
WO 2001/027061 and the references cited therein describe another process for preparing 2-methyl-3-(4-tert-butylphenyl)propanal.

EP 0 638 665 and the references cited therein describe the synthesis of the diacetal of the formula VII, which can be converted via hydrolysis to the corresponding aldehyde of the formula IX, where the diacetal of the formula VII is obtained via electrochemical oxidation of 4-tert-butyltoluene of the formula III.

The 2-methyl-3-(4-tert-butylphenyl)propanal prepared as in WO 2001/027061 usually comprises about 0.5 to 2% by weight of the corresponding meta-isomer of the formula II, namely 2-methyl-3-(3-tert-butylphenyl)propanal, based on the total mass of the compounds of the formulae I and II, since the 4-tert-butyltoluene used as starting material comprises about 1 to 5% by weight of 3-tert-butyltoluene of the formula IV, based on the total mass of the compounds of the formulae III and IV.

DE 102 23 970 describes the use of a partitioned column for distillative purification of 2-methyl-3-(4-tert-butylphenyl)propanal of the formula I from a crude mixture, whereupon a 2-methyl-3-(4-tert-butylphenyl)propanal with less than 1.0% by weight content of impurities was isolated.
The presence of from 0.2 to 70% by weight content of 2-methyl-3-(3-tert-butyl-phenyl)propanal (formula II) in the product mixture has not hitherto impaired the fragrance properties of 2-methyl-3-(4-tert-butylphenyl)propanal (formula I). C. Sell, Angew. Chem. 2006, 118, 6402-6410, describes 2-methyl-3-(3-tert-butyl-phenyl)propanal (formula II) as having a more intensive odor than 2-methyl-3-(4-tert-butylphenyl)propanal (formula I). A certain content of 2-methyl-3-(3-tert-butyl-phenyl)propanal (formula II) has therefore actually been desirable in fragrance compositions comprising the abovementioned lily-of-the-valley fragrances used in perfume applications.
However, a change in the requirements of the market has made it necessary to reduce the content of auxiliary components in 2-methyl-3-(4-tert-butylphenyl)propanal (formula I), in particular 2-methyl-3-(3-tert-butylphenyl)propanal (formula II), and the content of undesired 2-methyl-3-(3-tert-butylphenyl)propanal (formula II) in the 2-methyl-3-(4-tert-butylphenyl)propanal (formula I) prepared by the processes described above, usually more than 0.5% by weight, based on the total mass of the compounds of the formulae I and II, is therefore significantly too high.